Most sewer systems have three major problems: (1) the back up of sanitary sewers into basements; (2) the back up of storm sewers into streets and onto tree banks and sidewalks; and (3) the overflow and diversion of sewage into streams and lakes. In communities which have separate storm and sanitary sewers, all of these problems except the flooding of streets can be solved by complete enforcement of laws prohibiting the connection of down spouts, foot drains, and sump pumps to be sanitary sewer.
Until Jan. 1, 1977, it was legal to divert overflow of wastes into streams and lakes. Only in the past ten years has it been necessary to concentrate on the solution of overflow and diversion problems.
The three major methods which up to the present time have been instituted, studied or proposed are: (1) the separation of storm sewers from sanitary sewers; (2) the storage of overflow for periods of low flow; and (3) the treatment of overflow so that it is safe to discharge into lakes or streams. The first method will accomplish the elimination of the overflow of interceptor sewers, but will not solve the street flooding problem caused by back up of storm sewers. In additon, the cost is prohibitive.
Method two will solve the overflow problem in combined sewer systems if the sewage treatment plant has the capacity to handle the overflow during periods of low flow. In addition, if the size of the storage facility is determined from overflow data which is extrapolated to the heaviest rainfall expected, it is subject to serious errors. The only accurate method of calculating maximum overflow due to surface water is to measure the size of the outlet of a catch basin and multiply the amount of water that it will carry by the number of catch basins feeding the interceptor sewer. This method is expensive, but not as expensive as separating sewers. In addition to its high cost, it will not solve the problem of street flooding or the back up of sewage into streets. Neither will it prevent basement flooding through sanitary sewers.
Method three will be less expensive than method two if a more efficient treatment of the waste is divised. Otherwise, it suffers the same drawbacks as method two. The elimination of overflow and backup of sewers by alteration of catch basin outlets appears never to have been previously considered, as is demonstrated by the following considerations: The government has lowered its standards for sewage treatment plant effluent because the only methods considered are storage and separation procedures. The separation procedure is out of the question because it is too expensive. The estimated cost of accomplished separation in existing combined sewer systems is estimated to be over $200,000,000,000 in the United States. Overflow measured directly and extrapolated to the largest storm in 100 years indicates that sewage plants are too small to handle the stored wastes even in periods of low flow. Also, the direct measurement of overflow would not be considered, nor would the 100 year storm be considered, if sewer system consultants were aware that any method or means were available for use in solving their measurement problems. No publications are now known relating to elimination of overflow and back up from sewers filled therefrom, and no publications are now known relating to control of flow from catch basins into sewer systems.
So far as is known to me, inexpensive effective techniques and means for overcoming the above indicated problems in combined sewer systems have not heretofore existed. Solution of these problems by my technique would be desirable not only from the standpoint of improving and overcoming faults with existing sewers, but also from the standpoint of making possible in the future further utilization of combined sewers in new construction situations which would avoid the inherently higher costs associated with separate storm sewers and sanitary sewers.